The use of a pulsed laser for the treatment of the surface of a metallic workpiece is described in a number of prior patents including U.S. Pat. Nos. 4,539,461, 4,755,237, 4,398,966, 4,684,781 and 5,209,791. These patents describe treatments utilizing the laser beam to effect a transformation, generally to a molten state and the quenching thereof. For example, U.S. Pat. No. 4,398,966 rapidly heats chromium nickel steel alloys to a solid solution temperature and then cools that structure at a rapid cooling rate.
A pulsed laser operating at a power of, say 250 to 350 watts, at a wavelength of 0.5 to 11 .mu.m with a pulse rate of about 100 to 200 pulses per second can be used. The depth of melting is say 0.5 mm with greater or lesser depth being selected as desired and a spot diameter being about 0.25 to 1.3 mm. The laser beam can be swept across the surface with a travel speed of up to 1.5 to 2 meters per minute. In this case, the body itself forms a heat sink for rapidly cooling down the surface when the beam leaves the heated spot.
In U.S. Pat. No. 4,539,461, the laser beam is used for hardening flank and root areas of gear teeth utilizing a nonlinear scanning technique. Back-tempering is prevented by directing cooling fluid onto gear tooth flanks opposite those currently being hardened. The cooling fluid can be liquid nitrogen. This latter patent recognizes that the high rate cooling of the laser-treated surface is advantageous and can utilize a liquefied gas to augment any natural cooling which may result by conduction and heat away from the laser-treated site.
Thus, while it has been recognized in the art that advantages can be gained by the rapid cooling of a surface region of a metal workpiece which is treated with a laser and especially a pulsed laser beam, the problem of providing a satisfactory degree of cooling and a satisfactory cooling rate to achieve an optimal transformation of a surface region of the metal workpiece into an amorphous state has remained. Indeed, none of the patents mentioned describes a method which can yield the cooling speed at the metal surface which is sufficient to obtain a continuous amorphous structure of the treated region.
Furthermore, regions heated by a pulsed laser beam tend to be rapidly oxidized and, since there is an overlap of between 30 and 50% of each treated spot by the next treated spot when the laser beam and the surface are relatively displaced, there are areas which are oxidized twice in each treatment.
While the laser beam treatments described above thus represent a significant advance in metallurgical operations, in practice the workpiece, which may be a tool, tends to suffer irregular wear in the treated regions.
When a liquefied gas is used to supplement the cooling in the manner described, the liquefied gas expenditure is considerable and frequently the treated surface is covered by a layer of "snow" resulting from condensed and solidified moisture which can obscure the treated surface and even prevent access of the laser beam to it.
The apparatuses which have been used for the earlier treatments also tend to be rather complex at least in part because of the need to train a laser spot on a very specific region while providing contact of liquefied gas with another region and eliminating the presence of the "snow", etc.